1. Field of the Invention
This invention generally relates to a multidirectional optical scanner for multidirectionally scanning a light-reflecting target, such as a bar code symbol, and/or the field of view of a detector and, more particularly, to an arrangement for, and a method of, generating a multidirectional scan pattern for reading randomly-oriented bar code symbols, as well as for changing the size and/or the shape of the scan pattern.
2. Description of Related Art
Targets having indicia of different light reflectivity, such as bar code symbols, have been scanned by directing light along an optical path to a symbol located in the vicinity of a reference plane lying generally normal to the optical path. The light reflected off the symbol is detected by a detector having a finite field of view. The symbols and/or the field of view have been scanned with many different scanning patterns in the reference plane. The pattern has been a single scan line, or a set of generally parallel scan lines extending in one direction lengthwise along the symbol. See U.S. Pat. Nos. 4,251,798 or 4,387,297. The pattern has also been two sets of intersecting parallel lines extending in two directions relative to the symbol. See U.S. Pat. No. 4,369,361.
In the case of randomly-oriented symbols, it has been proposed to produce a dense scanning pattern extending in multiple scanning directions in order to maximize the probability that the symbol will be quickly read, no matter what its orientation. Thus, conventional point-of-sale (POS) scanners of the type generally found in supermarkets that have been built into and underneath check-out counters, deflect a laser beam in several directions and read symbols oriented in different directions that move across the reference plane, i.e. in the plane of or slightly above the countertop. To deflect the laser beam, a central mirrored polygon surrounded by a ring of many auxiliary mirrors is mounted below the countertop, each auxiliary mirror corresponding to a different scan line. Such constructions occupy a great deal of space and are easily accommodated under a supermarket countertop where a large amount of space is readily available. See U.S. Pat. No. 3,978,317.
It has also been proposed to generate a Lissajous scanning pattern in the reference plane. A pair of scanning elements, one deflecting a laser beam in the X-direction, and the other deflecting the laser beam in the Y-direction, are sinusoidally driven simultaneously to form a closed curved scan pattern. Although curved, the Lissajous pattern includes an interior central portion characterized by generally linear scan lines essentially orthogonally intersecting each other to form an X shape, and exterior portions characterized by sharply curved scan lines having small radii of curvature. Since such lightly curved scan lines are not useful for symbol reading, they are typically cut off, usually by sizing the exit window through which the outgoing laser beam passes en route to a symbol so as to permit passage therethrough of only the interior central portion of the Lissajous pattern, but to block the exterior portions thereof.
In order to generate truly multidirectional scan patterns, the use of holographic scanners was proposed in an article entitled "Multidirectional Holographic Scanner for Point-Of-Sale Bar-Code Symbol Reader", published in Optical Engineering, Nov.-Dec. 1984, Vol. 23, No. 6, p. 784ff. A holographic disc was rotated at one speed, and a laser beam incident on the disc was reversely rotated at another speed. The resultant outgoing beam had multidirectional scan lines. However, in the context of designing bar code symbol readers which are hand-held, or mounted on a support surface, where the available space is at a premium, and where it is desired to keep the overall size, weight and volume of the reader to a minimum, holographic discs are not practical. The holographic disc of said article is 200 mm in diameter--too big to provide the compactness desired in many applications for portable scanners; has an optical path of 350 mm--too long for many applications; and has a scan angle of .+-.10.degree.--too inadequate to achieve the compactness and the length of the scan desired in many applications.
In infrared search systems, complex scan patterns were used very widely in the past. In those systems, for example, see U.S. Pat. No. 4,039,246, the field of view of a detector was accurately scanned across space at high speeds in a rosette-type pattern across the detector in a gyro-optical system mounted within a radiation-seeking missile. Convex and inclined planar reflectors were rotated to generate the scan pattern. However, such missile-mounted gyro-optical systems for detecting intruder aircraft in large areas of space were too big and complex to be used in bar code reader systems where the symbol is typically in a working range several inches, or, in some cases, several feet, from the system.
Other types of complex scan patterns were generated by prism scanners comprising a pair of optical wedge-shaped prisms positioned in front of a detector, each prism rotated relative to the other. A light beam passing through each prism was refracted at each airprism interface. However, chromatic and other optical aberrations were often encountered with refractive prismatic elements and were largely uncorrectable. Also, the prismatic elements were only useful in certain effective wavelength regions. It was difficult to produce a wide angle of deflection. The mounting of each prism on a rotating element, and the drive for each rotating element were complex. These drawbacks rendered the prism scanners particularly unsuitable for bar code symbol reading applications.
Other optical code readers of which the applicants are aware are U.S. Pat. Nos. 3,663,800; 4,282,431; 3,718,761; 3,684,867; 4,493,989.
In all of the above-described scan pattern generators, the size and configuration of the scan pattern were not readily changeable, and certainly not during scanning. Applications exist, particularly in bar code symbol readers, where it would be desirable to either manually or automatically change the size and/or configuration of the scan pattern either prior to, or during, use.